Pyrrolizidine Alkaloids from Senecio roseus and Senecio helodes

Two new 13-membered macrocyclic pyrrolizidine alkaloids, (13R)-13-hydroxyretroisosenine (2b) and (12S)-12-hydroxyretroisosenine (2c), have been isolat...
0 downloads 0 Views 158KB Size
Download PDF
1322

J. Nat. Prod. 1997, 60, 1322-1325

Pyrrolizidine Alkaloids from Senecio roseus and Senecio helodes† Ana-L. Pe´rez-Castorena,‡ Amira Arciniegas,‡ Agustı´n Castro,‡ Jose´ L. Villasen˜or,§ Rube´n A. Toscano,‡ and Alfonso Romo de Vivar*,‡ Instituto de Quı´mica, Universidad Nacional Auto´ noma de Me´ xico, Circuito Exterior, Ciudad Universitaria, Coyoaca´ n 04510, Me´ xico D.F., Mexico, and Instituto de Biologı´a, Universidad Nacional Auto´ noma de Me´ xico, Circuito Exterior, Ciudad Universitaria, Coyoaca´ n 04510, Me´ xico D.F., Mexico Received May 2, 1997X

Two new 13-membered macrocyclic pyrrolizidine alkaloids, (13R)-13-hydroxyretroisosenine (2b) and (12S)-12-hydroxyretroisosenine (2c), have been isolated from Senecio roseus and Senecio helodes. Their structures were established from spectral and chemical studies including 2D NMR. The hydrochloride of retroisosenine (2a‚HCl) was also isolated, and its absolute configuration was determined by X-ray diffraction analysis. Previous work on Senecio species (Asteraceae) has shown the presence of pyrrolizidine alkaloids (PAs),1,2 which exhibit hepatotoxic activity and a broad range of other pharmacological actions.3,4 Plants containing hepatotoxic PAs are considered a health hazard for both humans and livestock. This induced us to undertake a systematic study of Senecio species from the section Mulgediifolii, which are rich in PAs.5,6 In this paper we report the results obtained from the chemical studies on S. roseus Sch. Bip. and S. helodes Benth. Two collections of S. roseus were studied. The first afforded mulgediifoline (1),6 retroisosenine (2a),6 and the new compounds (13R)-13-hydroxyretroisosenine (2b) and (12S)-12-hydroxyretroisosenine (2c). The second collection gave 2a, 2b, and 2c, and, in addition, the hydrochloride of retroisosenine (2a‚HCl) was obtained. The known compounds 1 and 2a were identified by comparison with authentic samples.6 Compound 2b, [R]25D +116.4°, exhibited in its IR spectrum bands for hydroxyl and ester groups (3495 and 1732 cm-1). The HRFABMS gave a protonated molecular ion peak at m/z 352.1779 corresponding to the molecular formula C18H26NO6 [M + 1]+. The 1H-NMR spectrum (Table 1) of 2b was almost superimposable with that of retroisosenine (2a),6 differing only in the H-12, H-13, and CH3-19 signals. The H-13 signal was observed as a doublet at δ 4.15 indicating its gem relationship to one hydroxyl group. The chemical shifts of CH3-19 and H-12 signals (δ 1.28 s and 2.08 dq), as well as the multiplicity of the latter, suggested that these groups were in vicinal positions to the alcohol function. In the 13C-NMR spectrum (Table 2), the C-12 and C-13 signals showed downfield shifts (∆δ +5.0 and +35.9, respectively), and those of C-11, C-15, C-18, and C-19 presented upfield shifts (∆δ -2.8, -1.6, -2.5, and -5.8, respectively), when compared with the analogous signals of 2a. The acetyl derivative 2d confirmed the presence of the OH group and the saponification products of 2b (retronecine and 3a) corroborated its relationship with 2a. The structure of the new acid 3a was deduced from its spectral data (Tables 1 and 2) and a NOESY experiment, which showed the following

correlations: CH3-9 with H-4; H-3 with Me-8 and Me10; H-4 with CH3-9 and C-6 methylene; and CH3-8 with Me-10. The results of a NOESY experiment of 2b (Figure 1), indicated a “sandwich” conformation, similar to that described for retroisosenine (2a).6 The absolute configuration of C-13 was inferred from the NOE effects between CH3-18 and H-13, and those of H-12 with CH317 and CH3-19. Therefore, the new PA 2b corresponds to (13R)-13-hydroxyretroisosenine. Compound 2c, [R]25D +88°, differs from 2b in having the alcohol function at C-12. This was apparent from the 1H-NMR spectrum (Table 1), which showed the CH318 and C-13 methylene signals as singlets at δ 1.44 and 2.21, respectively. The proposed position of the hydroxyl group was in agreement with the 13C-NMR spectrum (Table 2), in which the C-12 and CH3-18 resonances had shifted significantly downfield from δ 49.4 and 11.9 to 81.4 and 25.3, respectively. The saponification of 2c gave retronecine6 and the new necic acid 3b, whose NOESY spectrum indicated an anti relationship between CH3-9 and CH3-10. Similar results were obtained from a NOESY experiment on 2c. The C-13 methylene signal correlated with the CH3-17 and CH3-19 signals, and the H-15a signal with the CH3-18 signal, and therefore the alcohol function, CH3-17, and CH3-19 are in the same side of the molecule of 2c. If we suppose that C-11 and C-14 have the same configuration as those of 2a and 2b, then C-12 should have a S configuration. Because no NOE effect was observed between the hydrogens of the basic and acidic moieties of the molecule, the new alkaloid 2c, should possess an unfolded conformation, which is different from those of 2a and 2b.

†

Contribution No. 1592 of the Instituto de Quı´mica, UNAM. Instituto de Quı´mica. Instituto de Biologı´a. * To whom correspondence should be addressed. Phone: (525) 6224412. FAX: (525) 6-162217. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1997. ‡ §

S0163-3864(97)00228-0 CCC: $14.00

© 1997 American Chemical Society and American Society of Pharmacognosy

Notes

Journal of Natural Products, 1997, Vol. 60, No. 12 1323

Table 1.

1H-NMR

Spectral Data of Compounds 2a‚HCl, 2b-2d, and 3a and 3b (500 MHz, CDCl3)a

proton

2a‚HCl

2bb

2c

2 3a 3b 5a 5b 6 7 8 9a 9b 12/3e 13a/4e 13b/4e 15a/6ae 15b/6be 17/8e 18/9e 19/10e

5.93 br s 4.57 d (15.5) 3.75 dt (15.5, 2.0) 4.08 m 3.03 dt (11.0, 9.3) 2.42 m 5.55 ddd (5.0, 2.5, 2.5) 5.09 br s 5.06 d (13.0) 4.23 ddd (12.5, 2.5, 1.0) 2.34 dquint (11.0, 7.0) 2.06 dd (12.5, 7.0) 1.94 dd (12.5, 11.0) 2.65 d (13.0) 2.59 d (13.0) 1.44 s 1.05 d (7.0) 1.42 s

5.86 br s 3.94 dddd (15.9, 3.5, 1.9, 1.6) 3.45 ddd (15.9, 6.1, 1.7) 3.29 m 2.65 ddd (10.2, 8.4, 8.1) 2.07 m 5.45 ddd (4.6, 3.2, 2.1) 4.36 br s 4.93 d (11.7) 4.18 br d (11.7) 2.08 dq (9.9, 6.9) 4.15 d (9.9)

5.88 s 4.07 br d (16.8) 3.48 dd (16.8, 5.3) 3.46 m 2.69 m 2.18 m 5.34 br t (4.5) 4.51 br s 5.27 d (12.0) 4.16 dd (12.0, 1.3)

2.69 d (12.7) 2.54 d (12.7) 1.44 s 1.01 d (7.0) 1.28 s

2.66 d (12.5) 2.45 d (12.5) 1.46 s 1.44 s 1.60 s

2.21 s

2db,c

3ab,d

3bd

5.93 br s 4.22 m 3.64 m 3.64 m 2.88 ddd (12.0, 9.6, 6.5) 2.22 m 5.54 br td (4.2, 1.4) 4.74 m 5.15 d (12.2) 4.19 d (12.2) 2.34 dq (9.6, 7.2) 2.18 dq (10.4, 6.9) 5.36 d (9.7) 3.95 d (10.4) 2.25 d (13.5) 2.09 d (13.5) 2.72 d (13.1) 2.90 d (16.3) 3.05 d (15.0) 2.59 d (13.1) 2.74 d (16.3) 2.69 d (15.0) 1.31 s 1.44 s 1.38 s 1.04 d (7.1) 1.04 d (7.0) 1.18 s 1.49 s 1.23 s 1.46 s

a Assignments are based on COSY, long-range HETCOR, COLOC, HMQC, and HMBC experiments. b 2a was run at 200 MHz, and 2d and 3a were run at 300 MHz. c Ac signal at δ 2.11, s. d 3a in (CH3)2CO-d6 and 3b in CH3OH-d4. e Numbering of 3a and 3b.

Table 2. 13C-NMR Spectral Data of Compounds 2a‚HCl, 2b-2d, and 3a and 3b (75 MHz, CDCl3)a carbon

2a‚HCl

2b

2cb

2d

1 2 3 5 6 7 8 9 10/1d 11/2d 12/3d 13/4d 14/5d 15/6d 16/7d 17/8d 18/9d 19/10d Ac

132.2 s 128.1 d 59.6 t 53.8 t 34.2 t 72.2 d 77.9 d 58.7 t 172.3 s 86.9 s 44.4 d 46.7 t 81.7 s 47.0 t 168.7 s 24.9 q 14.4 q 30.3 q

133.2 s 132.7 d 62.0 t 53.9 t 34.9 t 74.0 d 77.7 d 59.9 t 172.5 s 84.2 s 49.4 d 81.5 d 81.6 s 45.5 t 169.2 s 24.7 q 11.9 q 24.9 q

133.1 s 131.3 d 61.6 t 53.7 t 34.5 t 74.1 d 77.3 d 60.1 t 173.4 s 89.2 s 81.4 s 52.3 t 80.2 s 46.9 t 169.6 s 20.8 q 25.3 q 30.5 q

132.1 s 131.4 d 60.6 t 53.4 t 34.5 t 73.2 d 77.1 d 59.5 t 171.7 s 84.1 s 47.7 d 81.7 d 81.3 s 45.6 t 168.8 s 24.4 q 11.9 q 25.0 q 170.4 s 20.8 q

3ac

3bc

174.9 s 84.7 s 49.3 d 81.2 d 82.2 s 44.0 t 174.5 s 25.9 q 13.2 q 22.8 q

177.6 s 91.7 s 81.5 s 51.6 t 82.7 s 47.9 t 173.4 s 20.1 q 24.9 q 28.1 q

a Assignments are based on DEPT, long-range HETCOR, COLOC, HMQC, and HMBC experiments. b Run at 125 MHz. c 3a in (CH3)2CO-d6 and 3b in CH3OH-d4. d Numbering of 3a and 3b.

Figure 2. ORTEP projection of 2a‚HCl (crystallographic numbering).

The present study on S. helodes has afforded retroisosenine (2a), (12S)-12-hydroxyretroisosenine (2c), and retroisosenine hydrochloride (2a‚HCl). The similarity in alkaloidal content of S. roseus, S. helodes, and S. mulgediifolius,6 is consistent with their inclusion in the same section (Mulgediifolii) of the genus Senecio. Compounds 1, 2a‚HCl, 2a, and 2b showed to be inactive against the Gram-negative bacteria Escherichia coli ATCC 10536, and the fungi Trichoderma viride and Aspergillus niger. The toxicity of these compounds in the brine shrimp (Artemia salina) assay was very modest (LC50 > 300 µg/mL). Experimental Section

Figure 1. Results of NOESY experiment of 2b.

The hydrochloride of retroisosenine (2a‚HCl), [R]25D +46°, exhibited 1H- and 13C-NMR spectra (Tables 1 and 2) very similar to those of 2a, except in the signals of H-3, H-5, H-8, C-1, C-2, and C-7, whose chemical shifts resembled those of oxyretroisosenine.6 The absolute configuration of this hydrochloride was determined by X-ray analysis (Figure 2). The presence of 2a‚HCl might have arisen because of the use of CHCl3 and CH2Cl2 during extraction, since these solvents produce HCl as a product of decomposition.7

General Experimental Procedures. Melting points were determined on a Fisher-Johns melting point apparatus and are uncorrected. Optical rotations were determined on a JASCO DIP-360 digital polarimeter. IR spectra were recorded on a Nicolet Magna-IR 750 spectrometer. EIMS data were determined on a JEOL JMS-AX505HA mass spectrometer at 70 eV. FABMS were obtained on a JEOL, JMS-SX102A mass spectrometer operated with an acceleration voltage of 10 kV, and samples were desorbed from a nitrobenzyl alcohol matrix using 6 keV Xenon atoms. HRMS measurements in the FAB mode were performed at 10 000

1324

Journal of Natural Products, 1997, Vol. 60, No. 12

resolution using electric field scans and poly(ethylene glycol) ions (Fluka 200 and 300) as the reference material. 1H-NMR and 13C-NMR data were obtained on a Varian Unity Plus 500, Varian Unity 300, and/or Varian Gemini 200 instruments. Chemical shifts were referred to TMS (δ 0). Standard Varian programs were used for COSY and NOESY spectra at 300 or 500 MHz. HETCOR and HMQC experiments were obtained for 1J CH )140 Hz at 75 and 500 MHz, respectively. Longrange HETCOR and HMBC experiments were obtained for nJCH ) 9 Hz at 75 and 500 MHz, respectively. Column chromatographies were carried out on Kieselgel G (Merck, Darmstadt, Germany). TLC was performed on Si gel 60 F254 (Merck). Plant Material. S. roseus Sch. Bip. was collected in Cofre de Perote, Veracruz, Me´xico, in December 1994 (MEXU 620273) and December 1995 (MEXU 722087). S. helodes Benth. was collected in the vicinity of Angangueo, Michoaca´n, Me´xico, in October 1995 (MEXU 528643). Voucher specimens are deposited at the Herbario del Instituto de Biologı´a, UNAM, Coyoaca´n, Me´xico D. F., Me´xico. Extraction and Isolation. Dried roots of S. roseus (343.1 g), collected in 1994, were extracted with MeOH. The extract was concentrated and stirred overnight at room temperature with aqueous 1 N HCl (200 mL) and Zn powder (34 g). The mixture was filtered; the solution, basified (NH4OH to pH 10) and extracted with CHCl3. Elimination of the solvent afforded 4.19 g of a residue that was purified by column chromatography using as eluent mixtures of Me2CO-MeOH (7:3, 3:2, and 1:1). Fractions eluted with Me2CO-MeOH (3:2 and 1:1) afforded 169.2 mg of 1.6 Fractions eluted with Me2CO-MeOH (7:3) were combined to give 1.91 g of a gum (fraction A). The dried and ground aerial parts (660 g) of S. roseus were extracted with MeOH. The extract was worked up in a manner similar to that described for the roots, affording 2.96 g of a residue that was purified by column chromatography using as eluent Me2CO-MeOH (7:3). As a result, 13.5 mg of 1 and 1.94 g of a gum (fraction B) were obtained. Fractions A and B were combined and purified by several column chromatographic separations using different mixtures of CHCl3-MeOH as eluents to give 1.01 g of 2a,6 199 mg of 2b, and 19.2 mg of 2c. Compounds 1 and 2a were identified by comparison with authentic samples of mulgediifoline and retroisosenine, respectively.6 Dried roots of S. roseus (750 g), collected in 1995, were extracted with MeOH. The solvent was evaporated and the residue stirred overnight with 2.5% aqueous H2SO4 (400 mL) and Zn powder (75 g). The mixture was filtered, and the filtrate was washed with CH2Cl2, basified (NH4OH to pH 10), and extracted several times with CH2Cl2 until it became Dragendorff negative. The first 10 extracts were combined and yielded 12 g of an alkaloidal mixture, as pale yellow crystals. The remaining extracts gave 2b (300 mg). The alkaloidal mixture was purified by consecutive column chromatographic separations using as eluent CHCl3-MeOH (19:1) and Me2CO-MeOH (7:3), affording 2a (3.85 g), 2b (1.25 g), 2c (60 mg), and 2a‚HCl (165 mg). The extraction of dried and ground leaves (1.198 kg) from the same collection, was carried out as previously described above for the roots. The alkaloidal extract (2.7 g) was purified

Notes

by repeated column chromatography yielding 2a (300 mg), 2b (730 mg), and 2c (48 mg). The leaves and roots of S. helodes (115 g) were extracted with MeOH. The extract was purified as described above for S. roseus. The alkaloidal residue (800 mg) was purified by repeated column chromatography using as eluent CH2Cl2-MeOH (19:1), affording 2a (261 mg), 2c (12 mg), and 2a‚HCl (18 mg). (13R)-13-Hydroxyretroisosenine (2b): white crystals from hexane-Me2CO; mp 187-190 °C; [R]25D +116.4° (c 2.01, CHCl3); IR (CHCl3) νmax 3495, 1732, 1452 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2; EIMS m/z 351 [M]+ (2), 336 (1), 307 (8), 236 (21), 136 (36), 120 (69), 119 (100), 93 (37); HRFABMS m/z found [M + 1]+ 352.1779 (C18H26NO6 requires 352.1760). (12S)-12-Hydroxyretroisosenine (2c): white crystals from hexanes-EtOAc; mp 167-170 °C; [R]25D +88° (c 2.0, CHCl3); IR (CHCl3) νmax 3600, 1726, 1460 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2; EIMS m/z 351 [M]+ (0.5), 307 (4), 264 (8), 220 (37), 138 (36), 136 (41), 120 (56), 119 (100), 93 (33); HRFABMS m/z found [M]+ 351.1672 (C18H25NO6 requires 351.1682). Retroisosenine Hydrochloride (2a‚HCl). Those fractions containing 2a were combined and washed with hot hexane, the insoluble part was dissolved in Me2CO and crystallized on addition of EtOAc to give 2a‚HCl: mp 215-220 °C; [R]25D +46° (c 2.0, CHCl3); IR (CHCl3) νmax 2291, 2229, 1739, 1460, 859 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2. A solution of 2a‚HCl (20 mg) in CH2Cl2 (5 mL, previously washed with base) was washed with aqueous NaOH (0.5 N, 3 × 2 mL), dried with Na2SO4, and concentrated to yield 17 mg of 2a, mp 118-120 °C. Acetylation of (13R)-13-Hydroxyretroisosenine (2b). Compound 2b (20 mg) was acetylated in the usual manner with 1 mL of pyridine and 1 mL of Ac2O. After 1 h, the reaction mixture was dried under an air stream. The residue was dissolved in CH2Cl2, washed with saturated NaHCO3, and then with H2O, dried, and concentrated, yielding 18.2 mg of 2d as a yellow oil: IR (CHCl3) νmax 1735, 1456 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2; EIMS m/z 393 [M]+ (2), 346 (2), 290 (13), 289 (13), 179 (13), 169 (32), 136 (54), 120 (78), 119 (100), 93 (48), 43 (44). Saponification of (13R)-13-Hydroxyretroisosenine (2b). Compound 2b (100 mg) and KOH (100 mg) in MeOH (10 mL) were refluxed for 3.5 h. The solvent was evaporated and the residue extracted with hot CHCl3 (10 × 10 mL). Elimination of the solvent gave a brown oil (40 mg) that by sublimation at 3 mm Hg and 100 °C yielded 7 mg of retronecine: mp 113-115 °C; [R]25D +45.4° (c 2.2, MeOH).10 The basic residue was dissolved in H2O, acidified with 2% aqueous H2SO4, extracted with Et2O, dried, and concentrated to give 22 mg of (4R)-4-hydroxy-cis-nemorensic acid (3a) from EtOAc: mp 172-174 °C; [R]25D +34° (c 2.06, MeOH); IR (KBr) νmax 3218, 1725, 1698 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2; EIMS m/z 232 [M]+ (0.4), 215 (1), 187 (100), 169 (4), 127 (30), 115 (16), 109 (13), 85 (18) 83 (14), 73 (13), 43 (48); HRFABMS m/z found [M + 1]+ 233.1030 (C10H17O6 requires 233.1025). Saponification of (12S)-12-Hydroxyretroisosenine (2c). Compound 2c (20 mg) and KOH (20 mg) in MeOH (5 mL) were refluxed for 3.5 h. The reaction

Notes

Journal of Natural Products, 1997, Vol. 60, No. 12 1325

Table 3. Atomic Coordinates [× 104] and Equivalent Isotropic Displacement Parameters [Å2 × 103] for 2a‚HCl (U(eq) is Defined as One Third of the Trace of the Orthogonalized Uij Tensor) Cl (1) C (1) C (2) C (3) N (4) C (5) C (6) C (7) O (7) C (8) C (9) O (9) C (10) O (10) C (11) O (11) C (12) C (13A) C (13B) C (14) C (15) C (16) O (16) C (17) C (18A) C (18B) C (19)

x

y

z

U(eq)

-4726 (3) 460 (16) -1010 (15) -402 (13) 1814 (10) 2130 (16) 3724 (15) 2949 (13) 987 (8) 2436 (15) 392 (15) 222 (10) -93 (13) -75 (14) -581 (17) 465 (9) -3030 (22) -3048 (24) -2932 (61) -1015 (20) -857 (17) 1250 (21) 2901 (13) 267 (20) -4257 (23) -4144 (71) -377 (26)

7568 (3) 4855 (9) 4260 (10) 2676 (10) 2409 (9) 901 (9) 1369 (10) 3003 (9) 2895 (5) 3772 (9) 6442 (8) 6203 (5) 7468 (11) 8805 (7) 7059 (9) 5600 (7) 6754 (15) 5311 (22) 5044 (40) 4433 (12) 2964 (12) 2701 (12) 2453 (9) 8328 (10) 6435 (26) 8123 (49) 4109 (13)

2242 (1) 1897 (2) 2102 (2) 2262 (2) 2125 (2) 1913 (2) 1618 (2) 1512 (2) 1299 (1) 1871 (2) 1723 (2) 1333 (1) 1127 (2) 1241 (2) 743 (2) 654 (1) 697 (3) 458 (5) 739 (11) 526 (2) 750 (2) 939 (2) 797 (2) 492 (2) 1054 (4) 904 (9) 148 (2)

83 (1) 59 (3) 70 (3) 75 (2) 58 (2) 73 (3) 85 (3) 60 (2) 59 (2) 60 (2) 72 (3) 64 (2) 66 (2) 117 (3) 64 (2) 67 (2) 112 (4) 108 (7) 103 (14) 95 (3) 107 (4) 80 (3) 110 (10) 128 (4) 126 (9) 146 (18) 227 (9)

mixture was worked up as described for the saponification of 2b, and afforded 8 mg of retronecine as brown oil, [R]25D +44° (c 1.96, EtOH)10 and 9 mg of (3S)-3hydroxy-cis-nemorensic acid (3b) as a yellow oil. The later compound exhibited [R]25D +24.2° (c 3.6, MeOH); IR (film) νmax 3196, 1728, 1713 cm-1; 1H-NMR data, see Table 1; 13C-NMR data, see Table 2; HRFABMS m/z found [M + 1]+ 233.1035 (C10H17O6 requires 233.1025). X-ray Diffraction Structure Determination for 2a‚HCl.8 Crystal data: C18H26ClNO5; crystal size (mm), 0.20 × 0.16 × 0.08; crystal system, orthorhombic; space group, P212121; unit cell dimensions, a ) 6.230 (1) Å, b ) 8.391 (2) Å, c ) 36.869 (7) Å; volume, 1927.4 (7) Å; Z ) 4; formula weight, 371.85; density (calcd), 1.281 mg/ m3; absorption coefficient, 1.968 mm-1; F(000), 792. Data Collection. Diffractometer used, Nicolet P3P; radiation, Cu KR (λ ) 1.54178 Å); temperature (K), 293 (2); monochromator, Ni-filter crystal; 2θ range, 3.0 to

115°; scan type, ω; scan range (ω), 1.4°; background measurement, stationary counter at beginning and end of scan, each for 50.0% of total scan time; standard reflections, three measured every 97 reflections; index range, 0 e h e 4, 0 e k e 9, 0 e l e 40 plus Friedel pairs; independent reflections, 2233; empirical absorption corrections (XABS 2).9 Solution Refinement. System used, SHELXL-93; solution, direct methods SIR92; refinement method, fullmatrix least-squares on F2; quantity minimized, ∑ω (F02-Fc2)2; absolute structure parameter, -0.03 (5); extinction correction, 0.0019 (3); hydrogen atoms, riding model, fixed isotropic U ) 0.08 Å-2; weighting scheme, ω-1 ) σ2(F02) + (0.0638P)2 where P ) (F02 - 2Fc2)/3; number of parameters refined, 239; final R indices (observed data), R1 ) 6.71%, ωR2 ) 13.10%; R indices (all data), R ) 11.76%, ωR2 ) 15.31%; goodness-of-fit on F2, 0.979; largest and mean ∆/σ, 0.004, 0.001; datato-parameter ratio, 9.343; largest difference peak, 0.19 eÅ-3; largest difference hole, -0.16 eÅ-3. See Table 3. Acknowledgment. We are indebted to Carmen Gutie´rrez Coutin˜o, Rube´n Gavin˜o, Isabel Cha´vez, Beatriz Quiroz, Hector Rı´os, Rocı´o Patin˜o, Claudia Contreras, Luis Velasco, and Javier Pe´rez for technical assistance. References and Notes (1) Robins, D. J. Nat. Prod. Rep. 1994, 11, 613-619. (2) Robins, D. J. Nat. Prod. Rep. 1995, 12, 413-418. (3) Hartmann, T.; Witte, L. In Alkaloids: Chemical and Biological Perspectives; Pelletier, S. W., Ed.; Pergamon: Oxford, UK, 1995; Vol. 9, Chapter 4, pp 155-233. (4) Roeder, E. Pharmazie 1995, 50, 83-98. (5) Romo de Vivar, A.; Pe´rez, A.-L.; Vidales, P.; Nieto, D. A.; Villasen˜or, J. L. Biochem. System. Ecol. 1996, 24, 175-176. (6) Romo de Vivar, A.; Pe´rez, A.-L.; Arciniegas, A.; Vidales, P.; Gavin˜o, R.; Villasen˜or, J. L. Tetrahedron, 1995, 51, 1252112528. (7) Miall, S. Diccionario de Quı´mica; Editorial Atlante: Me´xico D.F., Me´xico, 1943. (8) X-ray data for compound 2a‚HCl have been deposited in the Cambridge Crystallographic Data Centre and are available on request from Dr. Olga Kennard, University Chemical Laboratory, 12 Union Road, Cambridge CB2 1EZ, UK. (9) Parkin, S.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28, 53-56. (10) Culvenor, C. C. J.; Heffernan, M. L.; Woods, W. G. Aust. J. Chem. 1965, 18, 1605-1624.

NP9702289